1.Field of the Invention
This invention relates to a technology for forming halftone dots by comparing an image signal with threshold values assigned to individual pixels within each of replicated unit areas.
2. Description of the Related Art
Halftone dots are generally formed by use of a halftone dot signal generated by comparing a multilevel image signal with threshold values assigned to individual pixels within each of replicated unit areas. Two methods are available for defining the replicated unit area. In one method a halftone dot area forming a single halftone dot is used as the replicated unit area while in the other a large area including a plurality of halftone dot areas is used as the replicated unit area.
FIG. 1 is an explanatory diagram illustrating the method in which a single halftone dot area (also called a "halftone dot cell") is used as the replicated unit area. In this method the four corners of each halftone dot area coincide with corners of the pixel grid. This halftone dot area can therefore be replicated to tile the entire image surface. This method is, however, considerably restricted as regards the screen rulings and the screen angles that can be realized. This is because the four corners of each halftone dot area have to coincide with corners of the pixel grid.
FIG. 2 is an explanatory diagram illustrating the method in which a large area including a plurality of halftone dot areas is used as the replicated unit area. The illustrated example uses a area including 4.times.4 halftone dot areas as the replicated unit area. This type of replicated unit area including a number of halftone dot areas is called a "supercell." While the four corners of each supercell coincide with the corners of the pixel grid, the four corners of the individual halftone dot areas do not necessarily coincide with corners of the pixel grid. Since the supercell method is flexible regarding the number of halftone dot areas included in the supercell, it enables the screen rulings and the screen angle to be realized with greater freedom in the rational tangent method. The rational tangent method is a method of forming halftone dots so that the tangent of the screen angle is a rational number.
Ordinarily, however, the halftone dot areas in a supercell do not all consist of the same number of pixels; they generally include different numbers of pixels. FIGS. 3 and 4 are explanatory diagrams illustrating examples of prior-art supercells with a screen angle of 0 degrees and a screen angle of 45 degrees, respectively. In FIG. 3, the supercell includes a halftone dot area with 11.times.11 pixels, a halftone dot area with 11.times.10 pixels, a halftone dot area with 10.times.11 pixels and a halftone dot area with 10.times.10 pixels. Thus the halftone dot areas included in the supercell do not always have the same number of pixels. As a result, the number of lit pixels, or recorded pixels, in the halftone dots also differs between different halftone dots even when an image with a uniform halftone dot area rate is to be reproduced. This is even clearer from FIG. 4. Specifically, at a halftone dot area rate of 50% the supercell of FIG. 4 results in formation of halftone dots consisting of 5.times.5 pixels, halftone dots consisting of 4.times.5 pixels and halftone dots consisting of 5.times.4 pixels. The viewer may observe unevenness in such a halftone dot image.
Accordingly, since the individual halftone dots do not always consist of the same number of pixels in the supercell method, the prior art method is liable to produce unevenness in the halftone dot image.